FUNCTIONS OF LOCOMOTION. 69 



cording to their forms and structures : long bones, flat 

 bones, and short bones. 



As their name indicates, the long bones are elongated. 

 At each end they have an enlargement, head or epiph- 

 ysis, the interior of which is composed of spongy 

 tissue. Between these is the shaft of the bone or diaph- 

 ysis, made up exclusively of compact tissue, and hav- 

 ing a central canal, the medullary canal, in which is the 

 marrow. The structure of the long bones combines all 

 the conditions of lightness and strength, for these bones 

 act as levers for the great movements. The bones of the 

 upper arm and forearm, and those of the thigh and leg, 

 belong to this class. 



The flat bones have considerable surface ; they contain 

 practically no spongy tissue, and this accounts for their 

 great strength. They occur particularly in those parts 

 of the body where the bony system is required to protect 

 other organs : they form the envelope of the skull, the 

 pelvis, etc. 



The short bones are composed of spongy tissue in the 

 interior, covered by a thin layer of compact tissue. They 

 are not large, having about the same dimensions in all 

 directions, and several of them are usually arranged to- 

 gether in places where only small movement is required, 

 as in the wrist and the instep of the foot ; the vertebral 

 column is composed entirely of bones of this class. 



On the surfaces of the bones are found ridges or crests 

 for the insertion of the muscles, apophyses or condyles 

 for reciprocal articulation, and numerous holes that serve 

 for the passage of nerves and nourishing vessels. Their 

 tissue has great vitality, a proof of which is shown by the 

 rapidity with which the broken ends are mended in case 

 of fracture, by the formation of new tissue called callus, 



